A general structure of a semiconductor device 14 wherein a semiconductor chip 12 is mounted on a substrate 10 made of an electro-insulating material will be described with reference to FIG. 1 and FIG. 2(c) which is a sectional view taken along line A--A in FIG. 1.
In this regard, flip-chip bonding is a method for bonding the semiconductor chip 12 onto the substrate 10 while a surface of the former carrying active elements thereon is opposed to the substrate 10. In general, solder bumps 16 are formed as electrodes on the semiconductor chip 12 which is reversed upside down and positioned in place on the substrate 10, after which the solder bumps 16 are molten all together to connect the electrodes with connector terminals (not shown) formed on the substrate 10 and secure the semiconductor chip 12 on the substrate 10. Since the solder bumps are arranged not only on the periphery of the semiconductor chip 12 but also at any positions thereon, it is possible to easily obtain as many input/output terminals (I/O) as needed.
Also, since the semiconductor chip 12 is directly mounted on the substrate 10 via solder, there may be cases wherein an underfiller 18 (such as epoxy type resin) is filled in a gap between the substrate 10 and the surface of the semiconductor chip 12 carrying the active elements thereon to reinforce the bonding for the purpose of improving the reliability (strength) of the connecting portion.
When the flip-chip bonding is carried out, an anisotropic electro-conductive film or anisotropic electro-conductive adhesive having the same adhesive property as the underfiller may be used instead of the latter for bonding the semiconductor chip 12 to the substrate 10. Specifically, a semiconductor chip provided with Au stud bumps prepared by the Au wire-bonding and the substrate coated with the anisotropic electro-conductive adhesive or clad with the anisotropic electro-conductive film is prepared. The semiconductor chip is placed on the substrate via the anisotropic electro-conductive adhesive or the anisotropic electro-conductive film, and the assembly is heated under pressure to connect the semiconductor chip with the substrate. In this regard, the anisotropic electro-conductive adhesive or film contains nickel particles having a size of 3 .mu.m order in epoxy type resin and is cured by heat in the same manner as in the underfiller.
The above-mentioned prior art semiconductor device 14, however, has the following problems.
The substrate 10 or the semiconductor chip 12 is preferably of a square shape. This is because when a so-called constant-size substrate of a rectangular shape (including a square shape) is cut into individual square substrates 10, or when a sliced silicon wafer of a predetermined diameter is cut into individual square semiconductor chips 12, as many substrates 10 or semiconductor chips 12 as possible are obtainable with least waste, and also the square shape facilitates the patterning efficiency of circuit patterns formed thereon.
The semiconductor chip 12 is mounted onto the substrate 10 so that each of sides of the semiconductor chip 12 is parallel to the corresponding side of the substrate 10. In addition, generally, the semiconductor chip 12 is mounted onto the substrate 10 so that a center of the former coincides with that of the latter. See FIG. 1.
When the semiconductor device 14 is mounted onto an originally flat circuit board (not shown), the semiconductor device 14 itself is preferably of a flat shape to enhance the reliable connection between the circuit board and the substrate 10.
As described before, the connecting portion between the substrate 10 and the semiconductor chip 12 is reinforced, for the purpose of improving the durability or reliability thereof, by the underfiller 18 or the anisotropic electro-conductive adhesive or film which is formed of thermosetting resin and cured through a curing process. Actually, the substrate 10 often warps after the curing process.
This warpage phenomenon of the substrate 10 will be explained with reference to FIGS. 2(a) to 2(c) illustrating states prior to, during and after the curing process, respectively. While the explanation is made on a case wherein the underfiller is used as adhesive interposed between the semiconductor chip 12 and the substrate 10, the same is true to other cases wherein the anisotropic electro-conductive adhesive or film is used instead thereof.
First, in a state shown in FIG. 2(a) wherein the semiconductor chip 12 is merely placed on the substrate 10 prior to the curing process, no substantial warpage occurs both in the substrate 10 and the semiconductor chip 12. That is, an amount of warpage is approximately equal to that in the substrate 10 when it stands alone.
Next, during the curing process wherein the underfiller 18 filled in the gap between the substrate 10 and the semiconductor chip 12 is cured, the substrate 10 thermally extends at a high temperature. However, since the underfiller 18 is completely cured after the substrate 10 has fully expanded, no substantial warpage occurs also in both thereof even in this curing process. See FIG. 2(b).
Finally, in a passage wherein a temperature of the assembly is lowering to a normal temperature, the fully extended substrate 10 gradually contracts as the temperature lowers. Since an amount of contraction of a region B of the substrate 10 in which the semiconductor chip 12 is placed (bonded) (in other words, a region of the substrate 10 in contact with the underfiller 18) is smaller than an amount of contraction of the remaining region of the substrate 10 because the thermal expansion coefficient of the semiconductor chip 12 is smaller than that of the substrate 10. Accordingly, when viewed from a lateral side, a surface of the substrate 10 (a back surface) opposite to a surface thereof including the region B carrying the semiconductor chip 12 therein (a front surface) more contracts than the front surface, whereby the substrate 10 warps so that the back surface thereof is concave as shown in FIG. 2(c).
This warpage of the substrate 10; i.e., the warpage of the semiconductor device 14; has the following relationship with the region B carrying the semiconductor chip 12.
First, the warpage of the substrate 10 is a phenomenon caused by a fact wherein the region B of the substrate 10 in contact with the underfiller 18 could not fully contract to an extent corresponding to the original thermal expansion coefficient thereof, and therefore, the warpage occurs with the region B as a center; specifically, it occurs in the radial direction all over the substrate 10 from the central point of the region B. Assuming the warpage of the substrate 10 along an imaginary straight line L passing by the central point of the region B, there is a relationship in that the longer a segment of the straight line L in the region B, the more the warpage.
Second, assuming again that the warpage of the substrate 10 is along the imaginary line L, since the substrate 10 warps to be a generally U-shape as a whole with the region B as a center as mentioned above, the maximum displacement in the substrate 10 due to the warpage occurs at the intersection of the imaginary straight line L and a contour of the substrate 10 farthest from the region B. In the square-shaped substrate 10, the above-mentioned intersection farthest from the region B is resulted when the imaginary straight line L coincides with a diagonal line of the substrate 10; in other words, such an intersection exists at the respective corner of the substrate 10. That is, the maximum warpage generates between a pair of corners of the substrate 10 located opposite to each other along a diagonal line.
In the prior art semiconductor device 14, both of the substrate 10 and the semiconductor chip 12 are of a square shape, and the semiconductor chip 12 is mounted to the substrate 10 with the respective sides of the substrate 10 being parallel to those of the semiconductor chip 12 and with the center of the substrate coinciding with that of the semiconductor chip 12. Since the diagonal line of the substrate 10 coincides with that of the semiconductor chip 12, a length of the region B on the diagonal line, along which the warpage of the substrate 10 is maximum, is longest. Thus, it is likely that the generation of large warpage occurs at four corners of the substrate 10.